Transistor process and apparatus



March 3, 1959 D. v. GEPPERT TRANSISTOR PROCESS AND APPARATUSl 5Sheets-Sheet 1 Filed June 2, 1954 Si Si um 4m INVENTOR. 'zy/yer/ March3, 1959 D. v. GEPPERT TRANSISTOR PROCESS AND APPARATUS 3 Sheets-Sheet 2Filed June 2, 1954 ,6. mf @d y AU ,a n F. m WW ESA 30ml-:duly

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March 3, 1959 D. v. GEPPERT 2,876,184

TRANSISTOR PROCESS AND APPARATUS l Filed Juxyxe 2, 1954 5 Sheets-Sheet 3AAAAA @0 lh l PHOTO TUBE "RELAY l COI LS,

States Patent O TRANSISTOR PROCESS AND APPARATUS Application June 2,1954, SerialNo. 433,873 `21 Claims. (cl. zot-14s) The present inventionrelates to transistors, and more particularly to an improved process andapparatus for fabricating a transistor of the type presently referred toin the art as a surface barrier transistor.

Several types of transistors are presently known to the art. The earliertypes contain at least two forms of germanium or other semiconductorcrystal material. For example, the point-contact transistor contains asemi-conductor crystal of one form with modified regions of another formadjacent the points of the cat whisker emitter and collector electrodes.The earlier junction type transistor also includes a lsinglesemiconductor crystal of two different forms of crystal materialexhibiting n-p-n or p-n-p characteristics. The more recent alloyjunction ltransistor, usually formed by alloying an impurity metal suchas indium on opposite faces of an n-'type semiconductor crystal wafer,contains p-type recrystallized areas adjacent the alloyed metalelectrodes with respective p-n junctions within the crystal, so thatthis type also contrains two `forms of crystal material. A recentlydiscovered type of transistor referred to in the art as a surfacebarrier transistor, differs from the previous types in that it containsonly one form of semiconductor crystal material and has p-n junctionsformed on its opposite surfaces rather than internally. In the lattertype, the interfaces of the transistor that form the p-n junctions andperform the functions of emission and collection of the useful currentsare located on the surface of a uniform semiconductor crystal wafer suchas germanium. The latter construction permits accurate control `of thegeometry of the crystal vand provides improved performancecharacteristics. That is, a properly constructedy surface barriertransistor is capable of efcient operation and high power gain atfrequencies greatly in excess of the capabilities of the prior typesdiscussed above having internal p-n junctions.

Briefly, surface barrier transistors are formed by etching craters orcavities in opposite faces of a semiconductor crystal wafer, so that thethickness of the waferv between the cavities is reduced, for example, tothe order of about 0.0002. A metal is then electroplated in each of thecraters to form the emitter and collector electrodes. It is usual to usean n-type germanium wafer and -electroplate indium into the craters;however, zinc, cadmium, tin and other materials can also be used.

The etching and electroplating is usually carried out by means of anelectrolytic jet which is usually a salt 2,876,184 Patented Mar. 3,19,59

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cavities to form the emitter and collector electrodes. A 0.1 normalsolution of low pH has been found suitable for the jet, with a jetpressure of l5 p. s. i. It is to be noted that the current flow duringthe etching step is in the back direction of the p-n junction formedbetween the pet and the semiconductor. This current flow is saturationcurrent augmented by the effect of light which produces pairs ofcarriers in the barrier region. Therefore, high ambient light isdesirable during the etching step.

lt is essential for high power gain and for satisfactory high frequencyoperation, that the p-n junctions of a transistor be as close to oneanother as possible. It is, therefore, necessary that accurate andprecise control be exercised over the etching process to insure that anextremely thin layer or wall of the semiconductor crystal materialexists between the bottoms ofthe etched cavities to assure physicalcloseness of the junctions formed atthe bottom of these cavities.

It is also essential that junctions and, therefore, the electroplatedemitter and collector electrodes, be parallel with one another foreicient high frequency operation.

This presents a problem in the usual jet-etch process.

due to the tendency for the cavities to have dish-shaped or concavebottom surfaces. Unless steps are taken, the subsequent electroplatingfills the cavity so that the facing surfaces of the emitter andcollector electrodes are not parallel. It has been attempted after theelectroplating step to etch the electrodes so as to reduce theirdiameter and confine them to the bottom part of the dish-shapedcavities, but this etching is extremelyv diiiicult to carry out asutmost care must be taken that it does not completely destroy the platedelectrodes. Moreover, it is essential that the electrodes be directlyopposite one another, and desirable that the collector be somewhatlarger than the emitter, and this is diicult to control when etching isresorted to.

An object of the invention is to provide an improved process in whichopposing cavities are etched in the opposite surfaces of a semiconductorwafer andy by which the etching can be terminated when such cavities areseparated by an extremely thin barrier or wall of semiconductormaterial.'

Another `object of the invention is to provide such an improved processwhich may be carried out rapidly and etiiciently, and by apparatus thatuses readily available and relatively uncomplicated components.

solution of the metal to be electroplated. A current is passed throughthe jet and the semiconductor in one direction to effectuate etching andthe current is reversed to cause electroplating to take place. Forexample, the jet may contain indium salts (such as indium sulphate orindium chloride), and when the current is in the etching direction, thetwo sides o f the crystal wafer are etched to form cavities with a wallor barriery of semiconductor of a desired thickness therebetween. Whenthe desired thickness of the barrier isreached, the current is reversedand indium electrodes are plated in the t A further object of theinvention is to provide such an improved process and apparatus, by whichthe cavities are formed with essentially iiat bottoms and in Whichfmetalelectrodes are electroplated at the central, flattest portion of thebottoms to be essentially parallel with one another for improvedperformance in the resulting transistor. v

A feature of the invention is the provision of an irnproved process forfabricating a surface barrier transistor in which an electrolyticetching jet is directed to at least one surface of a semiconductorcrystal Wafer with a current being passed through the jet and the waferto effect the etching process, and in which the current through the jetis controlled in synchronism lwith a mechanical shifting of the crystalso that an etched cavity may be formed to a selected depth at oneposition on the surface of the crystal at the precise momentbreakthrough occurs at another position.

Another feature of the invention is the provision of such an improvedprocess in which the semiconductor crystal wafer is given furthermechanical movements during the etching step so thatthe etching jeteffectively scans a portion of the surface of the wafer to form anetched cavity having a relatively at and uniform bottom Suffa withlarger radial dimensions than cavities formed f 3 vwithout'suchscanning, and minated at the end of the etching step and the current is.reversed with the jet being directed at the center of 4the cavity sothat'an electroplated electrode may be disposed only on the'flat bottomsurface of the cavity, as is desired, and does not completely ll thecavity and extend up. the sides thereof.

The above and other features of the invention which areV believed to benew are set forth with particularity vin the appended claims, theinvention itself, however, together with further objects and advantagesthereof may `best be understood by reference to the accompanyingdrawings in which:

Fig. 1 is a schematicV representation of a transistor fabricated inaccordance with the surface barrier prin- Aciples;

Fig. 1A is a perspective view of such a transistor supported on a base;

Fig. 2 shows schematically the formation of etched and plated cavitiesin a semiconductor crystal wafer in accordance with the invention;

Fig. 3 is a fragmentary sectional view through a semiconductor crystalwafer assembly, processed in accordance `with the present invention;

Fig. 4 shows a system for the the formation of etched and platedcavities in a semiconductor crystal wafer in ,accordance with a furtherembodiment of the invention; l Figs. 5 and 6 are diagrams illustratingthe operation of the system of Fig. 4;

Figs. 7 and 8 show mechanical components of apparatus suitable forcarrying out the process of the invention; Fig. `9 is a detailed diagramof a suitable systemV for carrying out the improved process of theinvention;

Figs. 10 and ll are curves showing the waveforms of signals developed inthe system of Fig. 9; and

Fig. 12 is a diagram showing the motion produced on a semiconductorwafer by the system of Fig. 9.

In practicing the invention, an electrolytic jet is directed onto asurface of a semiconductor wafer, and an electric current is passedthrough the jet and the wafer to establish an etching action by the jeton the surface of the wafer. The electric current is modulated tocontrol the speed of the etching action,'and the relative position ofthe jet and the wafer is periodically varied in synchronism with themodulation of the electric current. This establishes the etching actionat two distinct positions on the surface of the wafer, and at differentspeeds at each position. In this manner, the etch-through at the highspeed position can be used as an indication of desired depth of theetched cavities at the low speed position.

' I n accordance with another aspect of the invention, theelectrolytic'jet is effectively made to scan each position in such amanner as to develop flat-bottomed cavities which have a larger radialdimension than those produced merely by directly the jet onto lthesurface without such scanning. The transistor of Fig. 1 includes asemiconductor crystal Wafer 10 which is usually composed of n-typegermanium although other semiconductor materials such as silicon can beused. The crystal wafer has'a pair of etched cavities formed on oppositesurfaces of the crystal and disposed directly opposite one another incoaxial relation. These cavities are separated by an extremely thinsemiconductor crystal layer or wall 11 and have metal electrodes 12, 13electroplated therein. Electrode 12,' for example, forms the collectorand electrode 13 the emitter `of the transistor. A `base electrode 14 isatxed to one end of the wafer 10, -in known manner. As previouslypointed out, it is essential for optimum performance characteristicsthat the cavities formed in the wafer k10 have relatively at andparallel bottom surfaces so thatthe metal electrodes maybe essentiallyparallel to one another, and that the separating wall or layer 11 vbeextremely thin.

Fig.v 1A shows-the semiconductorA 10V supportedon .a

in which thev scanning is terbase by the various leads extending fromthe emitter,

is supported in any known manner between a pair of,

opposing aligned nozzles 15, 16 directed at areas disposed on oppositesides of the wafer and preferably at the center thereof. Nozzles 15 and16 are connected by glass tubes 17 and 18v to a common line 19 having ametal section 19a and through which an electrolytic solution is passed.As previously noted, this electrolytic solution contains saltscorresponding to the metal that is to form the plated electrodes, which,for example, may be indium.

An electric lead 20 is connected to the metal portion 19a of line 19 toestablish electrical connection with thc solution, and a lead 21 iselectrically connected to the wafer 10. Leads 20 and 21 are connectedthrough a variable resistor 22 to a pair of center contacts on areversing switch 23. The reversing switch 23 is connected to a source 24of etching and plating current, and which current is modulated by asquare wave signal from source 24a in a manner to be described indetail. The square wave source 24a is also connectedl to anelectro-mechanical, transducer 25. Transducer 25 may be in the form of aspeaker and will be described in more detail hereinafter. The transduceris mechanically connected to wafer 10 by a wire 26 so that energizationof the transducer by the square wave from source 24a causes the wafer tomove back and forth in a plane pe'r pendicular to the axis of the jetsfrom nozzles 15 and 16. When switch 23 is actuated so that current flowsin the etching direction and the jets are initiated,-

' cavities are formed on the opposite sides of crystal 10.

' rapidly than that in the other pair.

The etching current is modulated by the square wave from source 24a insynchronism with the reciprocating movement of crystal 10 by transducer25, so that two pairs ofadjacent cavities are formed in the oppositefaces. One of the pairs of cavities corresponds to the maximumamplitudevalue ofthe etching'current with corresponding relatively high etchingspeed, and the other pair corresponds to the minimum value of theetching current with relatively low etching speed. In this manner, theetching in one of the pairs of cavities proceeds more It is possible, byterminating the etching process at the precise moment of breakthroughbetween the high etching speed cavities,`

to have the low etching speed cavities separated by an extremely thinregion or layer of the crystal material of accurately controlledthickness.

i Provision is made to de-energize square wave source 24a upon suchreversal and to maintain crystal 10 with the centers of the low etchingspeed cavities disposed between the jets. This causes the platedelectrodes 12 and 13 to be formed in the latter cavities. The resultingcon` iiguration is shown in Fig. 3, and this figure shows at A theplated electrodes 12 and 13 formed on the wafer 10 in the low etchingspeed cavitiesv 27, 28, with the high' etching speed cavities 29, 30forming a pilot hole at B. As previously noted, it is desirable that thecollector electrode be somewhat larger than the emitter electrode,

I' and this may be achieved by making one of the nozzles Aw=.2 -mil.is-thesame as the percent tolerance for Abriefly,therefore,- the .methoddescribed above consists m1mpart1ng, aflow frequency (about 3 C. P.-.S.) reciprothat the etching current at one position is slightly higher(about 5%, fon example) than the current in the other position. Thus,when the semiconductor is etched corn- .pletelythrough at the highercurrent position, the remaining rthickness of the -semiconductor-attheother position Qis extremelysmall. To .be morespecific, if W represents-the original thickness ofthe wafer (assuming it to be the, same. at Athe two positions to ibe etched), w the rem'aining thickness ofthe waferat the lower current positionat the momentthe waferis etched completelythrough atthe higher current position, Il. the` current at .the lowercurrent position, and .Iz thecurrent at the hlghercurrentlposition, itcan be shown that:

provided that the etching times at the two positions vare equal. -Forexample, for W=4 vmils and'I1= 0.95 I2, Furthermore, the percentltolerance for W In practice, the value of W will not be exactly ,thesame at-the twopositions, the etching times will not be v exactlygthesame at thetwo positions, and the current ratioswill `have. a tolerancewhich must be taken into .1, account. In, order to minimize ftheerrordue-to l unequal etching times, the square wave motion imparted to thewafer bytransducerfZS can be made extremely accurate by driving thetransducer by an electronic apparatus in a f manner to be described.In-order to minimize the error -due tothe current ratio, a circuit hasbeen devised and Awill .be'describedjherein which permits highlyaccurate -control of this ratio with little or no adjustment. order tominimize the errorduefto unequal values of W at thetwo positions,careful lapping and etching of-the :semiconductor wafer is required,andthe two etching g positions must be located very close to oneanother.

.The invention thus far described has been. found to solve oneoftheimportant problems discussed above. That is, it providesa simple andexpedient process for f etching the semiconductor wafer to provide anextreme- `ly thin base region with close tolerance betweenfthe-felectrloplated electrodes; for high power gain and high frequencyresponse in the resulting transistor unit. The

v'additional problem of forming the cavities so that the facing surfacesof the plated electrodes may beat and essentially parallel may be solvedby imparting aslight .circular motion to the wafer in addition to thesquare f wave motion.

This additional motion causesthe jets effectively toscan the cavitiesduring the etching process.

.Both the reciprocal rectilinear motion, produced by the square wavesand the circular motions should be in a plane perpendicular to the axisof the jets, andthis can be obtained by using a pair ofelectromechanical transducers mounted-in space quadrature and fedwithelectric .signals of, proper amplitude and phase.

Such arrangement is shown in Fig. 4 which is a View looking down on the`semiconductor wafer from above. Two transducers 2S, 25a are mounted inspace quadrature and .of any.y point on the crystal wafer will be asshown -in. -Fig.j 5. For example, if v the low-frequency square `.waveis .3 C. P. S. `andthe high frequencysine vwave-'is .60. CWP. .S., .thepoint on the .semiconductor under con- .sideration will rotatein acircle at a position A with an angular -velocity of` 60 R. P. yS. .andfor aduration Aof time equal .to 1/6 sec.. (or 1/2 theperiodof the lowfrequency ,square wave). .The point will then jump to position- B yandexecute a circleof the same diameter as the one at position A.The.pointalternatelyfperforms -a .total of.10. revolutions ateachposition. The diameter of the circles can -be easily setvbyadjusting theamplitude. of the sine wave signals, and the .distancebetween-the circles can be easily `set by adjusting the amplitude. ofthe square-wave.

Therefore, when such complex` motion isimparted-to the germanium wafer.relative to the jets, these iets .eiectively describe acircularscanning motion in the high etching. speed cavities 'and .in thelow etching speed cavities. When the motion is such that. the'scanningiscircular,. as shown in Fig. 5, the vshapeof theetched cavities is asshown in cross section in Fig. 6. To obtain atbottom cavities, it isonly necessary-to vary thediameter of the-circles periodically in such amanner that each elemental larea of the germanium in thecavities is`etched thesame length of time. Theoretically this -calls for bothfrequency .and amplitude modulation ofthe sine .wavesign'als It can beshown that the angular velocity w shouldvary with time t according tothe relation:

,tudexof' ,the sine wave signals in phaseawith the square @wave sothatthe diameter of the circle increases from zero to a maximum at eachposition A and BL This-synchronization is easilyaccomplished'-electronically.as willgbe yshown hereinafter. 'Fig. 3.illustrates` :the resulting improvement in the atness of the; pitbottoms '27, 28.

,To `realize the ideal geometry, of two parallel planar electrodes, thesine wave signals are terminatediat the instant the etching current isreversed to initiate plating.

(As previously noted the square wave is also terminated .at this instantand the wafer is positioned with the low etchcavities between the jets.)Thus the electrodesre plated out inthe exact center of the ilat bottompitsgor cavities. vBy using a large enough circular motionrelativeztothe jet diameter, only vthe bottoms and lnot,. the sides ofthe` cavities are plated, thus eliminating the n eed for Jdrasticchemicaletching 4of the plated region. That is, even though the sidesofthe cavities are dish-shaped, their increased radial'dimensions due toscanningduring the .etching process, allows the jets to be directed tothe center ofa relatively large area during plating..with-the platedportion of each cavity extending only over a portion of the at bottomsurface.

In order to accomplishthe. various required operations automaticallywhenfthe semiconductor etchesv completely through at the high etchingspeed pilot cavities, a light source is positioned on one side of thesemi-conductor, and a photo tube is positioned on the otherv side tocontrol an electronic system, in a manner to be described so that theetching is terminated and plating -initiated the instant there is abreakthrough between'v the pilot cavities.

' Various mechanical components for carrying outpthe y through the voicecoil produces rectilinear motion of thecoil andjof the wire 26 attachedthereto.

As shown in Fig; 8, the transducers or speakers 25, 25a are rigidlymounted in space quadrature on a pair of vertical metallic plates 35,36, the plates being disposed at right angles. Plates 35 and 36 have apair of aperturcs 37, 38 respectively formed thereinwthrough whichjfthesteel wires 26,2611 from the cones on 'the speakers 'f1.qjojec`ti`VThel size of the' holes in the steel plates is "chosen sofas to providecritical ornear critical damping jjtofthe' motionof the steel wires. Thesemiconductor `wafer10is attached to the junction of the wires in the'mannershown. An electrical connection is made to the crystal by 'a thinflexiblewire 39 corresponding to lead `21`1in Figs.v Zand 4. Theelectrolytic apparatusl is mounted' to 'be associated with the crystalin the manner y shown in Figs. 2 and 4. f Alight source is provided, andlight from that source 1 isfocsed on the pilot'cavity in crystal 10 bya'suitable 'convergence lens. A photo tube is mounted onthe opposite'side of the crystal to receive light from the light source and controlthe system in a manner to be described.` The mechanical arrangement forcarrying out the process ofthe invention is illustrated anddescribedherein in somewhat schematic form. However, it is believed that Atheactual physical relation of the various elements and'methods formountingthose elements will readily suggest themselves to those skilled in theart, and that adetailed explanation of the actual mounting details `vofl-the mechanical and physical arrangement is uncnecessary herein.

Aldetailed circuit for controlling the process described Y herein isillustrated in Fig. 9. The system ofFig. -9 includes an electrondischarge device 50 which is connected in well-known manner to-form ablocking oscillator. The cathode of device 50 is coupled to the cathodesof a pairof discharge devices 51, 52, the latter pairbeing connected toform a well-known Eccles-Jordan -bi-stable trigger circuit. lThe anodeof device 51` is kcoupled through a pair of normally closed contacts K2"to the control electrode of an electron discharge device 53.-'Device 5.3is connected` as a cathode follower driver .st'age','Y and its/:cathodeis connected through the voice coil of theelectromechanical transducer25 to a tap onibleeder 'Y 96 connected between B+ and ground. The anodeof v deviee`52 is coupled through a pair of normally closed vcontactsKz'" to the control electrode of an electron dis-l Device 55 is alsoconnected as a charge device 55.

v'cathodefollower driver stage, and its cathode is connected to the tapon bleeder 96 through the voice coil of the electromechanical transducer25a. The transf ducers 25 and 25a are mounted in space quadrature onmembers 35, 36 in thev manner previously described, andI mechanicallycoupled to the semiconductive wafer 10 by steel wires in the manner alsodescribed previously herein.

AThe blocking oscillator of device 50 is free-running, andy itoscillates withv a repetition rate of, for' example, about' pulses persecond. The blocking oscillator drives thebi-stable trigger circuit ofdevices 52, 52 and causes fthe trigger.v circuit to Vgenerate squarewaves at the anodes of the latter devices, these square waves having` arepetition frequency (for example) of three cycles per fsecond. Thesquare waves from the bi-stable circuit are applied in phase oppositionto the control electrodes of devices 53 and 55 so as to apply respectivesquare .V wave control signalsto the voicecoils of transducers 25,

" 25'a As previously`noted,"the application of these control signalscauses the transducersA to move the semiconductor wafer in a directionperpendicular to the electrolytic jet from nozzle 15 between a iirst anda second position with respect to the jet. The blocking oscillator ofdevice 50 has highly stable repetition characteristics so that thetrigger circuit of devices 51, 52 is ltriggered at a constant repetitionrate. This assures that the etching times at the two positions of thesemiconductor wafer will be absolutely equal.

A pair of diodes 57, 58 are connected with opposite polarity between thecontrol electrode of device 53 and respective sources of positive andnegative potential. The purpose of these diodes is to function asclippers and insure a good flat-top square wave control signal totransducer 25. Diodes 59 and 60 are included in the circuit of thecontrol electrode of device 55 for the same reason and to insure a goodflat-top square wave control signal to transducer 25a. The circuitsof'diodes 57-60 provide positive control of the amplitude'of the squarewave signals so that the semiconductor wafer is moved to be preciselycentered at each of its two positions and does not have a tendency todrift.

The anode of device 52 is also coupled to an electron discharge device61, the latter being connected as an amplifier. The anode of device 61is coupled to the control electrode of an electron discharge device v62.Electron discharge device 63 is connected -as a diode. Devices 62 and 63are connected in a power supply that supplies the etching and platingcurrents through the electrolytic jet and the semiconductor. Devices 62and 63 function in a manner to be described to modulate the currentthrough the jet in synchronism with the shift of the semiconductor bytransducers 25, 25a. This establishes a high speed etching action at oneposition of the transducer and a low speed etching action at the otherposition of the transducer.

The power supply includes a power transformer 64 having a primarywinding connected to the usual A. C. source, and having a secondarywinding connected to a usual rectifier bridge 65. Bridge 65 is connectedto a filter, which is connected to a series of ten volt voltageregulator tubes and one 75 volt regulator tube 69. The aforementionedfilter is comprised of iilter choke 77 and capacitors 74, 75 and 76. Thefilaments of devices 62 and 63 are energized by the secondary winding ofa filament transformer 78, the primary winding of this transformer beingconnected to the usual A. `C. source. The cathodes of devices 62 and 63are connected to a center tap on the secondary winding 78. Bridge 65 isconnected through a normally closed contact K1' to nozzle 15 toestablish electrical connection with the electrolytic jet, and thesemiconductor wafer 10 is connected to ground. v

Contact K1' is associated with a relay K1. RelayK1 also has a set ofcontacts K1", and when the relay is 'deenergized, contacts K1" connectthe cathodes of devices 62 and 63 to ground, and contacts K1 connectbridge 65 to nozzle 15. This condition of relay K1 establishes a currentilow through the jet and the semiconductor in a direction to produceetching by the jet` When K1 is energized, contacts K1 connect thecathodesl of devices 62, 63 to nozzle 15, and contacts K1 connect bridge65 to ground. This reverses the current flow throughthe jet and thesemiconductor to produce `electroplating. The control of relay K1 willbe described subsequently herein.

The square wave derived from the bi-stable trigger cir-V cuit of devices51, 52 is amplified in device 61 and applied to device 62 so that thelatter device may function as a modulator for the current supplied tothe spray and semiconductor. The square wavedrives device 62 betweensaturation and cutoff. When device 62 is driven to saturation, theplate-to-cathode voltage drop' is only a few volts, and thediode-connected device 63 is cut off. Therefore, when device 62 isdriven to saturation, the

'and Y' K2.

. by the blocking Loscillator.

connectedl to the control `electrodes of a pair of 'elec- -trondischarge devices I86, 87. These latter devicesare etching 'curren'tflows through device 62 andhas larelative- 1y high value determined :bythe voltage drop across the entire bank vof voltage regulator :tubes66-69. This voltage, for example, may be in the neighborhood vof 1575volts. When vdevice 62, on the other hand, is driven to cutoff, device63 -is conductive andthe current to the spray "and semiconductor ilowsthrough device 63. The etching current now has a relatively lower value,and depends upon the voltage vdrop across the voltage regulator tubesy'66-68 to the exclusion of the voltage drop across tube 69. This lattervoltage 'drop across tubes 66-68 is of the order, for example, of 1500volts. In the manner described above, the current through the jet andsemiconductor is controlled to have a relatively high value when thevsemiconductor is established in a first position by the squarewave-control -signals applied to transducers 25, 25a; and to have arelatively lower value when the semiconductor is established in a secondposition by the square wave control signals applied to the transducers.The use of the voltage regulator tubes assures constant voltage at eachof the modulated conditions, and this in turn assures a constant currentratio with good tolerance for the two etching positions of thesemiconductor wafer.

As previously described, the vcurrent is maintained in the etchingdirection until the jets break through the semiconductor at the first orpilot position. At this time, due tothe lower etching speed at thesecond position, a pair of oppositecavities are formed in thesemiconductor fat Vthe second position with a thin wall of semiconductor`material between the cavities.

As previously described, a photo-tube 80 is 'disposed adjacent thesemiconductor to receive light when breakthrough at the first position-of the semiconductor -10 occurs. The photo-tube transforms the lightinto electrical energy, and this energy is `amplified by an amplifierincluding ya pair of cascade-connected discharge "devices 81, 82. Theamplifier is constructed to amplify only alternating current signals, soas to `be unresponsive to ambient light falling on the photo-tube.However, when vbreak-through occurs, the light through the pilot hole isintermittently interrupted 'by the shifting Aof the semiconductor bytransducers l25, y25a. alternating current square wave which isamplified by This produces an the amplifier iof devices 81, 82 andapplied to a thyratron discharge device y83. The applicationof theamplified square wave on devicefres Vthat-device 'and' causes a currentflow through the lactuating -coils -of *relays IK1 The current flowthrough relay coil K1 immediately lreverses the current through the yjetand semiconductor from -the etch to rthe fvelectroplate direction.Moreoveig the current through the relay coil 'K2 causes contact K2' toopenv to remove the energizing potential -from the blocking oscillatorof device 50 to terminate the production of the square wave controlsignal. f

` electrode of device y55 at alixedfpositive potential. This establishesfixed lcurrents `through the respective voice coils of transducers 25and-25a, -and 'maintains the 'semi- 'conductor-10 in -its secondposition for the plating 'operationwith the jets being directed at lthecenter of the lcavities -formed at this position.

lA glow tube 85 is coupled to the blocking oscillator of device 50 tovbe actuated thereby, the -glow tube being connected as a relaxation'circuit 'for developing l-a -series of periodic exponential `waves in-response to triggering The -circuit 'ofi'tube 85 is connected as abalanced modulator, and fthe cathodes thereof are connected to -thefterminals of a center-'tapped -alternating Curreuttransformer, thecenter tap of which `ris grounded. The anodes of devices 86, 87 areconnected 575 in pushpull through a transformer 88, the primarywindingof the transformer being connected to the anodes lof the devices,and the center tap of f the primary winding being connected to thepositive terminal B-l--l'- throughfthe normally closed contact K2. Thesecondary winding of transformer 88 -is coupled to a pair of ampliiiersincluding discharge devices 89 and 91. This coupling is made through apair of resistance-capacity networks 92, 93 'which respectively advanceand retard the phase of the output signal from the balance modulator by45 as appliedto amplifiers 89, '91. Amplifier 89 is coupled through anoutput transformer 94 to the voice coil of transducer 25a, and amplifier91 is coupled through a transformer 95 tothe voice coil of transducer25.

The spiral motion of wafer 10 discussed previously herein is produced bymodulating the control electrodes of devices .'86, 87 by the exponentialsignal of device 85. The exponential signal is a fairly goodapproximation to the desired modulation wave form. The high frequencylsine wave signal (having a frequency of, for example, 60 C. P. S.) isfed to the cathodes of devices 86, :87 with a mutual 180 out-of-phaserelation. The balanced modulator prevents any low frequency modulationcomponents from appearing in its output circuit. The `output signal fromthe secondary of transformer 88 thus appears -as shown in Fig. 10. Aspreviously noted, this output signal is connected throughresistance-capacity phase shifting networks 92, 93 so that the controlelectrodes of `amplifiers 89, 91 are driven in phase quadrature.

The resultant signal applied to the voice coils of transducers 25, 25a,is as shown in Fig. 1l, this figure showing the composite wave formed bythe square waves from devices 53, 55 and the exponentially varying sinewaves from devices 89, 91. In the manner previously described, this waveform causes `the semiconductor to shift from a first 4position A to asecond position B recurrently, and in synchronism with the modulation ofthe current through the jet and semiconductor. In addition to thisshifting, the modulated sine waves cause the jets electively to scan aVspiral :path n each position to produce the desired at bottom lfor thecavities. The path taken by any point on the germanium wafer as a resultof these signals is shown in Fig. 12. Since the relaxation circuit ofdevice 8S is triggered by the blocking oscillator circuit of device 50,the modulation of the sine-wave signals -is synchronized with thesquare-wave control signals. Therefore, each shift of the `semiconductorwafer and the commencement lof each spiral Vscan'by the jets occurs atthe same time.

The invention provides, therefore, an improved method and yapparatus forforming metallic electrodes on the opposite surfaces of a semiconductorwafer, with the electrodes being yseparated by an extremely thin Wall ofthe semiconductor material. The process is rapid and vellicient in itsoperation and -may be fully automatic by using the 'system described inconjunction with Fig. 9. That system provides for the control of theetching current *so that well-'defined at bottom cavities are formedinthe opposite surfaces `of the wafer. At the precise moment ofbreakthrough of the jetsin the pilot hole, the Vetching-current isreversed to produce the electroplating action, fthe semiconductor =isreturned and held in the proper position with the cavities disposed inthe paths `of the jets, and the signals previously applied to thetransducers and lto the current modulator are cut olf.

Thepresent process and apparatus enables clean, parallel,ilat-bottomedcavi'tes to be formed in the opposite surfaces lof thesemiconductor, and the control is such that these cavities can Vbeformed to a depth wherein they -are 'separated only by an extremely thinbarrier or wall lof the semiconductor material.

This enables transistors having extremely high power gain and highfrequency response to be constructed.

' `have iiat parallel facing surfaces, without the necessity fora.drastic chemical etching process. f Y I While particular embodiments ofthe inventionhave been shown and described, modifications may be made,'and it is intended in the appended claims to cover all suchmodifications as fall within the true spirit and scope of the invention.v

Ifclaim: v n

' :.flrThe method for etching a cavity in at least one vsurface ofasemiconductor wafer which comprises, direct- -ing ian electrolytic yjetonto the surface of the wafer, passing lank electric current through thejet and through 'the -wafer to establish an etching action by the jet onthe surface 'of the wafer, periodically varying the relative 'positionsof=said jet'and said waferwith a reciprocal movement so that said jet isrepeatedly directedto at -'leasttwodistinct positions-on the surface ofthe wafer Iforsuccessive short periods of time, and etching away apredetermined .amount of the wafer each time the jet is fdirected fatoney positionY and etching away a lesser 'amount of the wafer than saidpredetermined amount each time: the jet yis directed at the otherposition.

. =2.1The method for etchingV a pair of opposing cavities Vin oppositesurfaces of a semiconductor Wafer which comprises, directing a pair ofaligned opposing electrolytic jets'. onto the opposite surfaces of thewafer, passing an Lelectric kcurrent'through the wafer and the jets toestab- .lishA etching actions by the jets on the respective surfaces [ofythe wafer, periodically varying the relative kpositions .ofisaid jets.and said wafer with a reciprocal movement along anaxis perpendicular tothe axis of said jets yso 4that said jets are each repeatedly directedto two distinct positions on the respective surfaces of the Wafer forsuccessive short periods of time, etching at one rate at one of saidpositions, and etching at a rate different from Ysaid one rate at theother of said positions.

3.* The method for etching a pair of opposing cavities -finoppositesurfaces Vof a semiconductor WaferY which comprises, directing a pair ofaligned opposing electrolytic `jetsl onto the opposite surfacesv of thewafer, passing an electric current through the jets and the wafertovestablish't-etching actions by the jets on the respective oppositesurfaces of the wafer, periodically varying the position of Vsaid-waferwith a reciprocal movement transversely of .fthe axis of said jets sothat said jets are each repeatedly directed to two distinctpositions ontherespective op- "posite surfaces of the wafer for successiveshortperiods ivoftime, changing said electric current through said jets fromahigher value in onelof said positions to a lower .value in the other ofsaid positions to change the `effective- `nessfof the etching action ofsaid jets in said respective positions, and terminating the etchingaction of said jetszuponbreakthrough of opposing cavities formed therebyin one of said positions whereby opposing cavities are -formed in theother of said positions separated by a 4-vlvall ,of the semiconductorwafer having a predetermined v thickness.

. 4.'The method forietching a cavity in at least one [surface kof asemiconductor wafer which comprises, directingan electrolytic jet ontothe surface of the wafer, passving an electric current through thejetand through the rwafer to establish an etching action by the jet onthe the jet and the wafer with a reciprocal-movement in Asynclzironismwith the change of said electric current to es'tablish the etchingYaction yintermittently at different positions onthe surface ofthe waferandat different etching speeds at at least two ofsuch positions.

5.` The method for etching a cavity in at least one surface of asemiconductor Wafer which comprises, directing an electrolytic jetonto-the surface of the semiconductor, passing a unidirectional electriccurrent of a selected direct-current level through the jet and throughthe semicondutor to establish an etching action by the jet on thesemiconductor, periodically switching said electric current between ahigher anda lower value by a rectangular wave to establish twoselectedspeeds for the etching action, and periodically varying the relativepositions of the jet and the semiconductor with a reciprocal-movement ina direction perpendicular to the direction of. the jet in synchronismwith the switching 'of said electric current to establish the etching-action intermittently at two different positions on the 'surface of thesemiconductor and at different etching speedsfor each such position.

- 6.k The method for-etching a pair of cavities on opposite sides of asemiconductor wafer, with-the cavities being mutually separated byr athin wall of the semiconductor, whichmethod comprises, directing alignedelectrolytic jets onto the respective surfaces of the opposite sides ofthe semiconductor, passing a unidirectional electric current `ofavselected direct-current level through the jets and through thesemiconductor to establish an etching action -by the jets. on thesemiconductor, periodically switching lsaid electric current between ahigher and a lower value byv a rectangular wave to establish twoselected speeds for the etching action, and periodically shifting thesemiconductor with a reciprocal .movement in a direction perpendicularto the axis of the jets between arst and `a secondposition insynchronism with the switching of 'the respective surfaces of theopposite sides of the semiymethod comprises, directing alignedelectrolytic jets onto conductor, passing a unidirectional electriccurrent of a selected direct-current level through the jets and throughthe semiconductor to establish an etching action by the jets on thesemiconductor, modulatingk said electric current by a rectangular waveto establish a high and a low speed yfor the etching action,periodically shifting the semiconductor with ya reciprocal movement in adirection perpendicular to the axis of the jets between a first and asecond position in synchronism'with the modulation of said electriccurrent to establish intermittently high speed etching action at saidrst position and intermittently low speed etching action at said secondposition, and terminating said etching action upon breakthrough of saidjets at said first position.

8. The method for etching a cavity in at least one surface of asemiconductor wafer, which comprises, directing an electrolytic jet ontothe surface of the semiconductor,

`passing-a unidirectional electric current of a selected direct-currentlevel through the jet and through the semiconductor to establish anetching action by the jet on the semiconductor, varying said electriccurrent between different selected direct-current levels numericallygreater than zero so as to control the speed of theetching action,periodically varying the relative positions of the jet and thesemiconductor with a reciprocalv movement in synchronism with thevariation of said electric current to establish the etching action atdilerent'positions on the surface of the semiconductor and at dierentetching speeds at at least two of such positions, and imparting ascanning motion between the jet and the semiconductor at at least one ofsuch positions. Y

9. ',Ihe method for etching a pair of cavities on opposite sides of asemiconductor wafer, with the cavities ibeings'eparated by a thinwall-of the semiconductor,

' of said electric cuir'entto establish the 'etching action v-at "saidrst'and second 'positions andat 'diie'rent speeds -for each of suchpositions, and 'imparting lav recurrent spiral scanning motion betweenthe'jetfand 'the semiconductor at at least one of such positions.

10. The method for yetcliiri'ga pair of cavities on opposite sides of asemiconductor'wafei 'and for electroplating -respective metallicelectrodes Vin such cavities, with the metallic electrodesbeingseparatedby'a thintw-all of the semiconductor, which lmethodcomprises directing opposing -aligned electrol-ytic' jets "of -aImetallic salt solu- `tion onto'the respective Asiirface`sfofthe'op'pos'ite sides of the semiconductor, passing a unidirectionalAelectricl eurrent of -a selected direct-'current'level through the jetsand through the semiconductor :in a direction to establish an etchingaction by the jets 4on fthe-semiconductor, modulating said electriccurrent by arectangular waveto establish va high and a'loW speed for'the etching action, -periodicali ly shifting the semiconductor'with areciprocal movement in a direction perpendicularto the axis of the jetsbetween a rst and a lsecond :position 'in syiihionism with themodulation of `said electric current to Sest'ablish the high speedetching action at said rrst position andthe low speed etching action atsaid second position, and reversing-` the directioriof said currentponbreakthrough -of said jets at said iirst vpositionuto deposit a metallicelectrode by 'electroplating at said second position on each side of thesemiconductor. t l

1`1. The 'method foretching'a'pai'r'fof cavities ontop- 'posite sidescfa semiconductor wafer and for 'electroplating respective metallicelectrodes ,in suchc'avities, with the metallic electrodes beingseparated by a thin wall of the semiconductor, which method comprises,directing opposing aligned electrolytic jets of a metallic salt solutiononto therespective surfaces of the opposite sides of the semiconductor,passing a unidirectional electric current of a selected direct-currentlevel through the jets and through the semiconductor in a direction toestablish an etching action by the jets on the semiconductor, modulatingsaid electric current by a rectangular wave to establish a high and alow speed for the etching action, periodically shifting thesemiconductor in a direction perpendicular to the axis of the jetsbetween a rst and a second position in synchronism with the modulationof said electric current to establish the high speed etching action atsaid rst position and the low speed etching action at said secondposition, imparting a recurrent spiral scanning motion between the jetand the semiconductor at said first and second positions, terminatingthe periodic shifting of the semiconductor and the scanning motion andreturning the semiconductor to said second position upon breakthrough ofsaid jets at said first position, and reversing the direction of saidcurrent upon such breakthrough to deposit a metallic electrode byelectroplating at said second position on each side of thesemiconductor.

12. Apparatus for etching a cavity in at least one surface of asemiconductor wafer, including in combination, a device for forming anelectrolytic solution into a jet, mounting means for positioning thesemiconductor in the path of the jet with a selected surface thereoffacing the jet for impingement thereby, an electrical circuit connectedto said device and to the semiconductor for establishing an electriccurrent through the jet and semicon.

14 duct'or in a direction to establish an etching action between the jetand the semiconductor, a modulator in said electric circuit and'responsive to 'an applied signal for modulating said electric current'to control the speed of the etching action, actuating means coupled tosaid mounting means and rresponsive to an applied signal forperiodically varying the 'relative positions of the jet and thesemiconductor, and means for producing a periodic signal rand forapplying the same to said modulator and to said actuating means toestablish 'the etching action at diiferent positions on .the surface ofthe semiconductor and at different etching speeds at at least two vofsuch positions. l

'13. Apparatus -for etching a cavity in at least onesurface of asemiconductor wafer, including in combination, a device for forming anelectrolytic solution into a jet, an electromechanical transducer`mechanically coupled to the semiconductor wafer to position the waferVin the path of the jet with a selected surface of the wafer facing thejet for impingment thereby, and said transducer being responsive to anapplied signal for shifting the wafer in a direction perpendicular ktoythe axis of the jet, anelectric circuit connected to said=device and tothe semiconductor for establishing an electric current through the jetIand semiconductor in a direction to establish an etching action betweenythe jet and the semiconductor, a modulator in said Aelectric vcircuitand "responsive to an applied signal for modulating said electriccurrent to Ycontrol the speed of the etching action, and means forproducing an electrical signal having a rectangular wave shape'and for'applying the same to said modulator and to said electromechanicaltransducer to establish the etching action -attwo'difrerentrpositionsfon the surface of the semiconductor and atdiieren't etching speeds at each of such positions. l

' 14. Apparatus defined in claim 13 in which said `electromechanicalItransducer comprises a loud speaker having a voice coil' movablymounted thereon for rectilinear 'motion in response to anfappliedsignal, and a rigid wire mechanically coupled to said voice coil andextending along the axis of motion thereof to supportthe semiconductoratthe end thereof remote from said voice coil.

15. Apparatus defined in Yclaim 13 in which said electrical signalproducing means comprises a free running blocking oscillator, and abi-stable trigger circuit to said blocking oscillator and actuatedthereby.

16. Apparatus for etching a pair of cavities on opposite sides of asemiconductor wafer and for electroplating respective metallicelectrodes in such cavities, with the metallic electrodes beingseparated by a thin wall of the semiconductor, said apparatus includingin combination, a device forming an electrolytic metallic salt solutioninto a pair of opposing aligned jets, an electromechanical transducermechanically coupled to the semiconductor wafer to position the wafer inthe paths of the jets with opposite surfaces of the wafer facing therespective jets for impingement thereby, and said transducer beingresponsive to an applied signal for shifting the wafer in a directionperpendicular to the axis of the jets, an electric circuit connected tosaid device and to the semiconductor for establishing an electriccurrent through the jet and semiconductor in a direction to establish anetching action between the jet and the semiconductor, a modulator insaid electric circuit and responsive to an applied signal for modulatingsaid electric current to control the speed of the etching action, meansfor producing an electrical control signal having a rectangular waveshape and for applying the same to said modulator and to saidelectromechanical transducer to establish a high speed and a low speedfor the etching action and to establish the low speed etching action ata lirst position of the semiconductor and the high speed etching actionat a second position of the semiconductor and control means responsiveto the breakthrough of said jets at said second position for reversingthe direction of said current to l deposit-a metallic electrode byelectroplating at said rst positionson each side of the semiconductor.

17. Apparatus defined in claimr 16 in which said control meansadditionally terminates the production of said control signal, andapplies a xed potential to said transducer to maintain the semiconductorin said first position.

18. Apparatus delincd in claim 16 in which said contolmeans includes aphoto tube, and a light source di rected towards said photo tube througha point on the semiconductor that is ybrought into alignment with saidphoto vtube when-the semiconductor is in said second position. .Y

19. Apparatus for etching a pair of cavities on opposite sides of asemiconductor wafer and for electroplating respective metallicelectrodes in such cavities, with the metallic electrodes beingseparated by a thin wall of the semiconductor, said apparatus includingin combination,

a. device `for forming an electrolytic metallic salt solution into apair of opposing aligned jets, a pair of electromechanical transducersmechanically coupled to the semiconductor wafer to position the wafer inthe paths of theijets' with opposite surfaces ofthe wafer facing therespective jets for impingement thereby, andvsaid trans ducers beingresponsive to an applied signal for shifting thewafer perpendicular tothe axis of the jets, means for mounting said transducers in spacequadrature relation cto ,provide rectilinear motion for the Wafer alonga pair of axes vdisposed at right angles to one another, an electriccircuit connected to vsaid device and tothesemiconductor'forestablishing an electric current through the iet andsemiconductor in a direction to esablish an etchingaction between thejet and the semiconductor, a mod- 4ulator in said electric circuit andresponsive to an applied signal for modulating said electric current tocontrol the speed of the etching action, means for producing anelectrical control signal having a rectangular waveshape and forapplying the same to said modulator and to said electromechanicaltransducers to establish a high speed anda low speed for the etchingaction and to establish the, low speed at arst position of thesemiconductor and the high speed at asecond position of the semiconductor,means for producing aperiodic exponential Wave synchronized withsaid control signal, a modulator for '.modulating a sine wave with saidperiodic ,exponential Wave, said sine wave having a relatively highfrequency compared with said control signal, a network for applying `themodulated sine wave fromk said last named modulator to said respectivetransducers in phase quadrature relation, andV control means responsiveto the break- .through of said jets at said second' position forreversing kdirected, to two alternate positions on the surface of thewafer to etch progressively two adjacent cavities therein, and stoppingthe etching action of the jet when the jet rst breaks through the Wallof one of said cavities.

21. The method of etching a cavity having an essentially flatbottom ofva predetermined area in a surface of a semiconductor wafer-,which methodincludes they steps of directing an electrolytic jet onto the surface ofthe wafer, the jet having a smaller cross-sectional area than the areaof thebottom of the cavity, applying an electric vcurrent through thejet and the wafer to establish an `etching action by the jet` andimparting a spiral scanning motion between the jet and the wafer withconstantly decreasing increase of radius of the spiral.

References Cited in the file of this patent UNITED .STATES .PATENTSBailey May 23, 1922 2,721,834 Koury Oct. 25, 1955 2,744,860 Rines May 8,1956 2,746,918 Whittington May 22, 1956 OTHER REFERENCES Proceedings ofThe Institute of Radio Engineers, December 1953, vol. 41, 'No. 12, pages1706 thru 1708; article by Tiley etal.

1. THE METHOD FOR ETCHING A CAVITY IN AT LEAST ONE SURFACE OF ASEMICONDUCTOR WAFER WHICH COMPRISES, DIRECTING AN ELECTROLYTIC JET ONTOTH ESURFACE OF THE WAFER, PASSING AN ELECTRIC CURRENT THROUGH THE JETAND THROUGH THE WAFER TO ESTABLISH AN ETCHING ACTION BY THE JET ON THESURFACE OF THE WAFER, PERIODICALLY VARYING THE RELATIVE POSITION OF SAIDJET AND SAID WAFER WITH A RECIPROCAL MOVEMENT SO THAT SAID JET ISREPEATEDILY DIRECTED TO AT EAST TWO DISTINC POSITION ON THE SURFACE OFTHE WATER A PREDETERMINE AMOUNT OF THE WAFER EACH TIME THE JET ISDIRECTED AT ONE POSITION AND ETCHING AWAY A LESSER AMOUNT OF THE WAFERTHAN SAID PREDETERMINED AMOUNT EACH TIME THE JET IS DIRECTED AT THEOTHERPOSITION.